This invention relates, in general, to phase modulated digital signals, and more specifically, to recovery of data from phase modulated digital signals.
Phase correction of incoming digital signals is performed through various methods including Costas loops and other phase locked loops and remodulator schemes. Nearly all of these methods generate an RF signal within a voltage controlled oscillator (VCO) and compare the VCO generated RF signal with an original carrier input signal. If the generated RF signal is out of phase with the input signal (phase error), a phase locked loop locks the frequency and phase of the generated RF signal with the frequency and phase of the input signal. The phase lock loop varies the VCO output proportional to a phase error determined within the phase locked loop. This process is the basis of the Costas loop, the "XN" loop, and the remodulator loop.
The frequency and phase of phase demodulators which incorporate phase locked loops must be adjusted to match the frequency and phase of the input signal before demodulation can occur. Therefore, relatively long periods of time are required to acquire the phase of the input carrier signal.
Time delays resulting from phase locked loop phase demodulators create serious problems with signals transmitted in short bursts (e.g. time domain multiple access (TDMA) signals). Unless the phase error is removed quickly, information within short burst signals may be lost over long acquisition time or time delays. Furthermore, time delays create high overhead in the system due to long preambles attached to the data to facilitate phase acquisition. Therefore, a shorter time period for removing phase error is desirable, and in certain circumstances, required to allow quick demodulation.
Some receiving systems require easy adjustment for reception of varying signal types (e.g. BPSK, QPSK, MSK, 8PSK), or varying data rates. Phase correctors which utilize RF processing schemes require RF bandwidth and/or special harmonic generator circuits which can be cumbersome to adjust. Baseband processors, however, incorporate low pass filters which are easy to adjust. Baseband filters are, therefore, preferable over RF and IF filters when varying RF signals are to be received.